1) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device having an ohmic electrode formed on compound semiconductor having a wide band gap and its manufacture method.
2) Description of the Related Art
As an ohmic electrode for semiconductor having a wide band gap such as gallium nitride (GaN), a laminated electrode of titanium (Ti) and aluminum (Al) has been used conventionally. After the laminated electrode of Ti (lower layer)/Al (upper layer) is vapor-deposited on the surface of GaN, rapid thermal annealing (RTA) is performed to form ohmic contact. With RTA, a surface oxide layer slightly left on the surface of GaN is thermally destructed so that recombination centers are formed at a boundary between metal and semiconductor. Carriers are transported via recombination centers so that ohmic contact can be obtained.
As the impurity concentration of GaN is raised, a depletion layer formed in the surface layer of GaN becomes thin. As the depletion layer becomes thin, tunneling current can be increased in addition to hot electron field emission by which carriers are transported over the potential barrier at the junction. In this manner, ohmic contact with a small contact resistance can be formed.
The current-voltage characteristics at the junction between semiconductor and metal are determined by electron affinity, band gap, impurity concentration, respectively of semiconductor, work function of metal and the like. As metal is made in contact with semiconductor, there is a general tendency that the Fermi level of metal is pinned in the band gap of semiconductor. As metal is made in contact with semiconductor having a wide band gap such as GaN, the Schottky barrier at the junction between metal and semiconductor becomes high. From this reason, there is a limit in reducing contact resistance.
In forming ohmic contact on GaN, it is necessary to perform annealing at a high temperature of about 600 to 800° C. after a Ti electrode is formed, because oxygen left on the surface of GaN is strongly coupled to Ga.